Reduction of hydride losses from molten baths containing hydrides



Feb. 10, 1920 A, z ETAL 3,494,793

REDUCTION OF HYDRIDE LOSSES FROM MOLTEN BATHS CONTAINING HYDRIDES Filedlday 11. 1 966 1/1 avail; a

IN V N TOR:

United States Patent 3,494,793 REDUCTION OF HYDRIDE LOSSES FROM MOLTEN BATHS CONTAINING HYDRIDES Arnold Lenz, Cologne, Stammheim, and Walter Rogler,

Ranzel, Germany, assignors to Dynamit Nobel Aktiengesellschaft, Troisdorf, Germany, a corporation of Germany Filed May 11, 1966, Ser. No. 551,176 Claims priority, application Germany, May 12, 1965, D 47,234 Int. Cl. C23g 1/14 US. Cl. 134-2 1 Claim ABSTRACT OF THE DISCLOSURE Process for descaling metal surfaces comprising contacting the metals surface with an alkali metal hydride containing bath containing as the solvent for the hydride a eutectic mixture of sodium hydroxide and potassium hydroxide while maintaining the baths surface in contact with an inert dry shielding gas.

This invention relates to a method for the descaling of metal surfaces by pickling in hydride-containing baths which contain as the solvent for the hydride a eutectic or nearly eutectic mixture of sodium hydroxide and potassium hydroxide. More particularly, this invention relates to an improved method for the descaling of metal surfaces by pickling in hydride-containing baths which contain as the solvent for the hydride a eutectic or nearly eutectic mixture of sodium hydroxide and potassium hydroxide, in which the decomposition of the hydride at the baths surface is substantially prevented.

A process for descaling metal surfaces has been disclosed in copending application Ser. No. 451,671, now abandoned, in which the pickling is carried out with hydride-containing molten baths which contain as the solvent for the hydride a eutectic or nearly eutectic mixture of sodium hydroxide and potassium hydroxide. These baths can be used for the descaling of steel and non-porous metal articles of many different kindsi.e., wires and strips.

One particular advantage associated with this process is in connection with the replacement of the consumed hydride--i.e., the hydride can be added to the melt in the form of lumps of sodium hydride coated with a eutectic or nearly eutectic mixture of sodium hydroxide and potassium hydroxide, or else in the form of lumps consisting of a homogeneous mass of the eutectic material in which the sodium hydride is embedded.

In carrying out the descaling process, however, considerable losses occur which are to be attributed to the fact that the hydride decomposes on contact with the surrounding, and usually humid, air. These losses can be reduced, for example, by employing mechanical covering means, such as floating pans, hinged lids, airlocks, and the like, but their use does not entirely prevent the hydride losses since contact between the surface of the bath and the humid air cannot be entirely avoided, especially in the areas where the workpieces to be descaled enter or leave the bath. Protective measures for preventing access of moist air to the uncovered areas of the melt where the workpieces are immersed into the bath and removed again therefrom, as heretofore known, require relatively high technical expense and are not efficient to the degree required.

It is an object of the present invention to substantially eliminate hydride losses from pickling baths containing the same by contact of the bath surface with humid air.

Another object of the invention is the prevention of access of humid air to the surface of a pickling bath con- 3,494,793 Patented Feb. 10, 1970 taining an alkali metal hydride in a simple and inexpensive manner.

These and other objects will be apparent from a consideration of the following disclosure:

In accordance with the invention, it has now been found that, in the process for descaling metal surfaces wherein the pickling is carried out with hydride containing baths which contain, as solvent for the hydride, a eutectic or nearly eutectic mixture of sodium hydroxide and potassium hydroxide, the decomposition of the hydride at the surface of the bath can be prevented in a relatively simple manner by shielding the bath surface with an inert dry gas.

The process of the present invention makes it possible in a simple and most inexpensive manner to prevent access of moist air to the baths surface.

In order to obtain the necessary shielding, the conventional apparatus and measureswhich are well-known in the art of ventilation can be utilized.

As inert shielding gas for use in the invention, there may be employed those dry gases which do not enter into any chemical reaction with the hydride-containing baths prepared with the eutectic mixtures of sodium hydroxide and potassium hydroxide under the conditions of operation. Such gases include, for example, hydrogen, nitrogen, noble gases, oxygen, air, and saturated aliphatic hydrocarbons having up to 4 carbon atoms. It has proved advantageous in accordance with the invention to adjust the said gas to such a degree of dryness that their water vapor partial pressure amounts to less than about 5 torr.

In accordance with the invention, the uncovered areas of the melt, particularly where the wire or metal articles enter or leave the same, are shielded with an inert dry gas to prevent a reaction of the sodium hydride contained in the bath with the moisture content of the air which would result in unnecessary hydride losses. The inert gas can be introduced adjacent the point of introduction of the wires, strips, or the like so that the gas sweeps the atmosphere from the exposed surfaces of the melt. Gas conduits can be provided for this purpose. Other exposed areas of the melt, such as at the sides, can be swept in a like manner. Dry nitrogen has proved particularly well suited as the gas in the shielding.

The invention is illustrated by the following example of certain preferred embodiments thereof, although it will be understood that the example is illustrative only and not intended to limit the scope of the invention unless otherwise specifically indicated.

EXAMPLE In a small steel tank having a 2-liter capacity and a diameter of 200 mm., 2 kg. of a eutectic mixture of sodium hydroxide and potassium hydroxide having a sodium hydride content of 2 wt. percent was fused and raised to a temperature of 230 C. After 25 minutes of standing in the uncovered tank, the hydride content had fallen from 2 to 1% of weight.

The same procedure was again carried out with the sole difference that air, having a dew point of 35 C., was blown onto the surface of the bath in such a quantity that a velocity of flow of 30 cm./sec. with reference to the cross-section of the tank resulted. The hydride content of the bath was found analytically to have decreased to only 1.7 wt. percent after 25 minutes.

The shielding of the bath surfaces with an inert dry gas can be conducted, for example, with an apparatus to be described below.

The figure shows a section through the pickling apparatus parallel to the direction of travel of wires 3 or other elongated metal form. The pickling apparatus consists first of a trough like, elongated vessel 1, which is filled 3 with the fused NaOH-KOH bath 2 containing sodium hydride. The hydride content of the melt is not less than about 1 nor more than about 3% by weight, averaging 2% by weight. Position 2a marks the level of the melt. Heating devices, which are necessary for the production of the required working temperatures of about 200 to 240 C., have been omitted for the sake of simplicity. The wires 3 which are to be pickled are introduced continuously from left to right, for example, over ramps leading down into the apparatus, and are guided over a rotatable cylinder 4. Cylinder 4 is mounted in a roller column'4a. The melt 2 is covered on both sides of the roller column 4a by floating pans 5 which can be moved by means of hoists 6. The floating pans 5 are intended to prevent the access of moist air to the surface of the melt. The uncovered areas of the melt, especially where the Wires enter or leave, are expediently shielded with an inert, dry gas according to the present invention to prevent a reaction of the sodium hydride with the moisture content of the air, which would result in unnecessary hydride losses. An inert gas can be introduced adjacent the point of introduction of the wires so that the gas sweeps the atmosphere from exposed surface of the melt. Gas conduits 16 are provided for this purpose. Other exposed areas of the melt, such as at the sides where the pans 5 are adjacent the wall of the vessel, can be swept in a like manner. Dry nitrogen has proven particularly well suited for the shielding. Underneath the ramps are located, at the points of entry and departure of the wire, cage-like solution boxes 7. The cage 7 has at one side an elevator 8 in which a charging basket can move up and down. The charging basket is suspended from an appropriate hoist 6a and in its upper portion holds ballast While the lower portion serves for charging fresh hydride in lump form into the melt 2. The bottom portion of the charging basket is closed 01f at the top by a perforated, inclined cover plate 13. The space between the cover plate 13 and the bottom of the upper portion of the charging basket is empty. The lower portion is open towards the cage 7 and toward the adjacent wall of the vessel 1. The shaft has an opening for loading charging the basket with hydride. Under the influence of the ballast the charging basket can be brought into the unloading position. Since the hydride has a lower specific gravity than the NaOH-KOH melt, the hydride tends to float and rises slowly upward. Due to the slant of the walls 13 and 14, the lumps containing hydride enter into the portion of the cage 7 which is defined by the horizontal perforated plate. The pieces of hydride then slowly pass into solution.

We claim:

1. In a process for descaling metal surfaces wherein the metal to be descaled is contacted with alkali metal hydride dissolved in a melt of sodium hydroxide and potassium hydroxide in about eutectic proportion, the improvement which comprises maintaining the bath surfaces in contact with inert dry air having a water vapor partial pressure of less than about 5 torr as shielding gas and the bath at a temperature of from 200 to 240 C.

References Cited UNITED STATES PATENTS 2,134,457 10/1938 Tainton 134-l5 XR 2,188,930 2/1940 Vincent et al 134-42 XR 2,353,026 7/1944 Gilbert 134-29 XR 2,377,876 6/1945 Gilbert 134-2 2,796,366 6/1957 Carter 134-2 3,126,301 3/1964 Faler 134-2 XR MORRIS O. WOLK, Primary Examiner SIDNEY MARANTZ, Assistant Examiner U.S. Cl. X.R. 

